The present invention generally relates to a method and an apparatus for the inspection of a printed circuit board (PCB) assembly by machine vision means.
Automated inspection of surface mount devices (SMDs) on printed circuit boards is becoming more important as electronics devices get smaller and packing density gets higher. Automated inspection has better performance than manual inspection in terms of consistency, speed and lower cost in the long run.
The commonly found defects on a surface mount PCB assembly include missing components, misalignment, tilted components, tombstoning/open circuit, wrong components, wrong value, bridging/short circuit, bent leads, wrong polarity, extra components, lifted leads, insufficient solder, and/or excessive solder among other things.
An industrial useful inspection machine must be capable of detecting these defects reliably and fast enough to cope with the production rate. Broadly speaking, the inspection techniques can be divided into three different classes. These are the X-ray technique, laser scanning technique and machine vision technique. The X-ray technique consists of a fine focus X-ray source and appropriate detector to acquire and analyze X-ray images of the PCB assembly. Solder related defects, such as insufficient solder, excessive solder, and poor wetting, can be easily detected by the X-ray technique. However, wrong value, wrong components, wrong polarity, and bent leads cannot be inspected at all. U.S. Pat. Nos. 4,926,452, 5,097,492 and other related patents assigned to Four Pi Systems Corporation disclose inspection systems utilizing the X-ray laminography techniques. U.S. Pat. No. 5,012,502 assigned to IRT Corporation also uses the X-ray method to inspect the degree of interconnection of solder joints.
The laser scanning technique makes use of a laser triangulation method to determine the three dimensional (3D) profile of the PCB assembly. A laser beam is scanned across the surface of the PCB assembly, and the retuned beam information is used to calculate the 3D profile of the scanned path. The 3D profile of the PCB assembly is compared with that of a known good board. Wrong value and polarity cannot be inspected using this method. A small deviation from this method is the utilization of interference fringes created by the interference of two coherent laser beams on the surface of the PCB assembly. U.S. Pat. No. 5,414,513 assigned to Northdrop Grumman Corporation discloses such a method.
The machine vision technique makes use of images captured by cameras or another video source for the analysis of the PCB assembly. This technique, therefore, relies heavily on the lighting and viewing setup. U.S. Pat. Nos. 5,245,421 and 5,260,779 assigned to Control Automation of USA disclose a hemispherical lighting fixture containing hundreds of individually programmable LEDs which can be configured to achieve many combinations of lighting modes. Different solder or assembly defects are highlighted by different combinations of lighting modes. However, this method has certain limitations. For example, the light intensity generated by LEDs is usually not sufficiently bright, and thus the camera aperture needs to be wide open. A large camera aperture will limit the depth of the field of the imaging system. The detection algorithms are closely related to the lighting mode and hence the selection and programming of the hundreds of LEDs is difficult. Hence, the teaching time required for a new PCB assembly is long and complicated.
U.S. Pat. No. 5,185,638 assigned to International Business Machines Corporation discloses a computer controlled, multiple angle illumination system for inspecting defects on a PCB. The illumination system is computer controlled as to a level of intensity and adjustable as to an angle of incidence. The illumination system includes illumination control electronics, a quad quartz halogen lamp array light source, a fiber optic line converter and an illumination collection system. The viewing arrangement is not described. The desired inspection system should be reliable, accurate, immune to ambient conditions changes, and easy to set up for a new PCB assembly. The inspection system should be able to operate by ordinary technicians or operators with a minimum of programming or machine vision knowledge.
The present invention discloses a method and an apparatus that accomplishes most of these desired features, in particular, the importance of lighting and a viewing technique overcoming the deficiencies of known methods and apparatus.